[PATCH v2 2/2] rust: uaccess: use NonEmpty in strcpy_into_buf()

[Lorenzo Delgado <lnsdev@proton.me>]

`strcpy_into_buf()` returns `EINVAL` for an empty buffer and later
writes the NUL terminator through `last_mut().unwrap_unchecked()`,
relying on that earlier check.

Take the buffer as a `NonEmpty<[u8]>` instead, so the requirement is in
the signature. The `EINVAL` case goes away, and `last_mut()` returns
the element directly, which removes the `unsafe` block. There are no
callers to convert. A caller with an array buffer can pass it through
`NonEmpty::from_array_mut()` without a runtime check.

The `MaybeUninit` cast still starts from `[u8]`, so its SAFETY comment
does not depend on the layout of the wrapper.

Suggested-by: Gary Guo <gary@garyguo.net>
Link: https://lore.kernel.org/rust-for-linux/DM0D9TERQAM3.2Y3HV6P68E2N8@garyguo.net/
Link: https://github.com/Rust-for-Linux/linux/issues/1262
Signed-off-by: Lorenzo Delgado <lnsdev@proton.me>
---
 rust/kernel/uaccess.rs | 15 +++++----------
 1 file changed, 5 insertions(+), 10 deletions(-)

diff --git a/rust/kernel/uaccess.rs b/rust/kernel/uaccess.rs
index 5f6c4d7a1a51..1cc5c7580d3a 100644
--- a/rust/kernel/uaccess.rs
+++ b/rust/kernel/uaccess.rs
@@ -11,6 +11,7 @@
     error::Result,
     ffi::{c_char, c_void},
     fs::file,
+    non_empty::NonEmpty,
     prelude::*,
     ptr::KnownSize,
     transmute::{AsBytes, FromBytes},
@@ -383,20 +384,16 @@ pub fn read_all<A: Allocator>(mut self, buf: &mut Vec<u8, A>, flags: Flags) -> R
     /// Read a NUL-terminated string from userspace and return it.
     ///
     /// The string is read into `buf` and a NUL-terminator is added if the end of `buf` is reached.
-    /// Since there must be space to add a NUL-terminator, the buffer must not be empty. The
+    /// Since there must be space to add a NUL-terminator, `buf` is a [`NonEmpty`] slice. The
     /// returned `&CStr` points into `buf`.
     ///
     /// Fails with [`EFAULT`] if the read happens on a bad address (some data may have been
     /// copied).
     #[doc(alias = "strncpy_from_user")]
-    pub fn strcpy_into_buf<'buf>(self, buf: &'buf mut [u8]) -> Result<&'buf CStr> {
-        if buf.is_empty() {
-            return Err(EINVAL);
-        }
-
+    pub fn strcpy_into_buf<'buf>(self, buf: &'buf mut NonEmpty<[u8]>) -> Result<&'buf CStr> {
         // SAFETY: The types are compatible and `strncpy_from_user` doesn't write uninitialized
         // bytes to `buf`.
-        let mut dst = unsafe { &mut *(core::ptr::from_mut(buf) as *mut [MaybeUninit<u8>]) };
+        let mut dst = unsafe { &mut *(core::ptr::from_mut::<[u8]>(buf) as *mut [MaybeUninit<u8>]) };
 
         // We never read more than `self.length` bytes.
         if dst.len() > self.length {
@@ -422,9 +419,7 @@ pub fn strcpy_into_buf<'buf>(self, buf: &'buf mut [u8]) -> Result<&'buf CStr> {
             // This means that we filled the buffer exactly. In this case, we add a NUL-terminator
             // and return it. Unlike the `len < dst.len()` branch, don't modify `len` because it
             // already represents the length including the NUL-terminator.
-            //
-            // SAFETY: Due to the check at the beginning, the buffer is not empty.
-            unsafe { *buf.last_mut().unwrap_unchecked() = 0 };
+            *buf.last_mut() = 0;
         }
 
         // This method consumes `self`, so it can only be called once, thus we do not need to

-- 
2.55.0